1. Field of the Invention
The present invention relates to a method for determining in situ earth stresses and pore pressure and in particular to a method in which the overburden stress, vertical effective stress, horizontal effective stress and pore pressure are estimated from well log data.
2. The Prior Art
The estimation or determination of pore fluid pressure is a major concern in any drilling operation. The pressure applied by the column of drilling fluid must be great enough to resist the pore fluid pressure in order to minimize the chances of a well blowout. Yet, in order to assure rapid formation penetration at an optimum drilling rate, the pressure applied by the drilling fluid column must not greatly exceed the pore fluid pressure. Likewise, the determination of horizontal and vertical effective stresses is important in designing casing programs and determining pressures due to drilling fluid at which an earth formation is likely to fracture.
The commonly-used techniques for making pore pressure determinations have relied on the use of overlay charts to empirically match well log data to drilling fluid weights used in a particular geological province. These techniques are semi-quantitative, subjective and unreliable from well to well. None are soundly based upon physical principles.
Effective vertical stress and lithology are the principal factors controlling porosity changes in compacting sedimentary basins. Sandstones, shales, limestones, etc. compact at different rates under the same effective stress. An effective vertical stress log is calculated from observed or calculated porosity for each lithology with respect to a reference curve for that lithology.
The previous techniques for determining in situ earth stresses have relied on strain-measuring devices which are lowered into the well bore. None of these devices or methods using these devices use petrophysical modeling to determine stresses from well logs. They are unsuitable for overburden stress calculations because the various shales hydrate after several days of exposure to drilling fluid and thus change their apparent porosity and pressure.
There have been many attempts to detect pore pressure using various physical characteristics of the borehole. For example, U.S. Pat. No. 3,921,732 describes a method in which the geopressure and hydrocarbon containing aspects of the rock strata are detected by making a comparison of the color characteristics of the liquid recovered from the well. U.S. Pat. No. 3,785,446 discloses a method for detecting abnormal pressure in subterranean rock by measuring the electrical characteristics, such as resistivity or conductivity. This test is conducted on a sample removed from the borehole and must be corrected for formation temperature, depth and drilling procedure. U.S. Pat. No. 3,770,378 teaches a method for detecting geopressures by measuring the total salinity or elemental cationic concentration. This is a chemical approach to attempting a determination of pressure. A somewhat similar technique is taught in U.S. Pat. No. 3,766,994 which measures the concentration of sulfate or carbonate ions in the formation and observes the degree of change of the ions present with depth drilling procedures being taken into consideration. U.S. Pat. No. 3,766,993 discloses another chemical method for detecing subsurface pressures by measuring the concentration of bicarbonate ion in the formation being drilled. U.S. Pat. No. 3,722,606 concerns another method for predicting abnormal pressure by measuring the tendency of an atomic particle to escape from a sample. Variations in rate of change of escape with depth indicates that the drilling procedures ought to be modified for the formation about to be penetrated. U.S. Pat. No. 3,670,829 concerns a method for determing pressure conditions in a well bore by measuring the density of cutting samples returned to the surface. U.S. Pat. No. 3,865,201 discloses a method which requires periodically stopping the drilling to observe the acoustic emissions from the formation being drilled and then adjusting the weight of the drilling fluid to compensate for pressure changes discovered by the acoustical transmissions.